Field of the Invention
The present invention relates to an illumination optical system, an exposure apparatus, and a method of manufacturing an article.
Description of the Related Art
In a photolithography technique that forms a fine pattern on a substrate by projecting a pattern formed on a mask onto a substrate to which a photosensitive material has been applied, the mask needs to be illuminated at a uniform illumination. In contrast, for example, there is known a technique that improves illumination uniformity by arranging an internal reflection optical integrator between a light source and a target illumination surface. Note that the internal reflection optical integrator can be a glass rod or a hollow pipe and is also called an optical pipe. These terms will be used interchangeably in this specification.
For example, Japanese Patent Laid-Open No. 7-201730 discloses a method that improves the illumination uniformity of a target illumination surface by arranging an exit end face of an optical pipe in a conjugate position with a reticle surface. Japanese Patent Laid-Open No. 2002-025898 discloses a technique that can effectively propagate light from a light source unit to a target illumination surface when the angular distribution (effective light source distribution) of the light to be condensed on the target illumination surface has an annular shape. However, in this illumination system, the shadow from an electrode wire included in the light source unit such as a discharge lamp or the cooling nozzle for suppressing heat emission of the light source unit are reflected on the effective light source when the light is condensed by the ellipsoid mirror. The loss of light amount caused by this can be a factor that avoids improving the productivity of an exposure apparatus.
On the other hand, Japanese Patent Laid-Open No. 2008-262911 discloses an arrangement which tries to prevent, as much as possible, the loss of light amount by integrating a cooling nozzle and a power supply cable of a light source unit.
Although the arrangement disclosed in Japanese Patent Laid-Open No. 2008-262911 has an arrangement in which the cooling nozzle and the power supply cable of a light source unit are integrated and can reduce the loss of light amount, a shadow that falls on the exposure light has an adverse effect on the effective light source distribution, and it becomes impossible to ignore its effect on imaging performance. Note that “the effective light source distribution” refers to a light intensity distribution on a pupil plane of an illumination optical system that illuminates a mask. Assume that, for example, the shadow of the integrated power supply cable and cooling nozzle falls on the exposure light and this appears on the effective light source distribution. Consider a case when a repetitive same-width pattern is arranged, on a mask, in a direction horizontal (X direction) to a predetermined direction of the shadow and in a direction vertical (Y direction) to the predetermined direction of the shadow. In this case, if the mask is illuminated with illumination light having the effective light source distribution affected by the above shadow, a difference between the X-direction line width and the Y-direction line width of the projected pattern is generated. This is because the transfer efficiency of diffracted light which holds pattern information changes depending on the direction.
In this manner, if the effective light source distribution is affected by the shadow of the power supply cable or the cooling nozzle, good imaging performance may be impossible depending on the direction of the pattern.